he promised that he would pay it back in a month
The interest rate is inconsequential compared to the risk of never seeing the original $2,000 ever again. You're not a bank and high-risk, negative reward is the usual outcome.
Turn this "promise" into a binding contract with a payment schedule and potentially collateral.
If you value their friendship then you will not charge interest for such a short-term loan.
If the other person values your friendship then they will pay back the entire principle within a timely fashion.
The reality is that a person who actually needs $2,000 and thinks they can comfortably pay it back in a month is not on level ground. Do they actually have a plan for getting caught up within one month's time? I'd estimate such a featAt $15/hour, they would need to require 20work 89 hours of overtime per weekin a month to earn $2,000; that doesn't sound sustainable to me. A realistic repayment schedule is $300-500 per month.
As a friend, I would have said that $500 is all I can offer for now. In my mind, I'd write it off as a gift so I can sleep at night but would hold them accountable for repayment.